The Three Flavors of HTML

To help in the transition from the era of using HTML tags that have evolved in an ad-hoc fashion to control the appearance of Web pages to a new era in which the appearance of Web pages will be controlled entirely by the use of styles sheets, the W3C has formulated three different flavors of HTML 4: strict, transitional, and frameset. These are defined in the form of three separate DTDs (Document Type Definitions). A DTD is the formal definition of a specification by the W3C. Thankfully, the W3C also provides less formal and more easily comprehended explanations in the form of the official specifications, meaning that you don't have to know how to read the DTD (no easy task in itself) in order to understand the specification. If you want to learn how to read the HTML 4 DTDs, for instance, the W3C provides instructions on its “On SGML and HTML” page at http://www.w3.org/TR/html4/intro/sgmltut.html.

However, you only need to know that these three versions of HTML, strict, transitional, and frameset, conform to three separate formal DTDs of the same name. You do need to know what they signify, however, if you're going to be using styles. If you want to validate your CSS style sheets, you'll also need to include a DOCTYPE declaration at the top of your document. A DOCTYPE declaration declares the type of document that is being coded.